This invention relates to transistor amplifier circuits, and more particularly it is directed to a transistor amplifier circuit of the cascode type.
As a transistor amplifier circuit having the gain control function, one of the cascode type is known. This amplifier circuit has the operating point and the bias current of an amplifier transistor kept constant even when a gain control signal AGC changes. It is accordingly capable of providing gain control over a wide range without bringing about changes in the dynamic range and the input impedance of the amplifier circuit.
This amplifier circuit, however, involves the problem that, where a resistance is connected to the emitter of the amplifier transistor in order to expand the input dynamic range, the signal-to-noise ratio of an amplified signal output is conspicuously degraded in dependence on the state of the gain control.
As a cascode type amplifier circuit to solve this problem, one is known which is disclosed in Japanese Patent Application Publication No. 6961/1970 entitled "Amplifier Circuit." According to this amplifier circuit, a resistance is incorporated on the emitter side of an output side transistor of a pair of differential transistors, and the gain of the differential amplifier circuit is lowered. Thus, improvements in the signal-to-noise ratio are intended.
It has been revealed, however, that the value of the resistance cannot be made very large due to restrictions of the bias current and the base potential of the differential transistor, so that noises beyond certain bounds cannot be prevented from entering by means of the resistance, resulting in degradation of the output signal-to-noise ratio.